The following relates to the magnetic resonance arts. It finds particular application in radio frequency coils for magnetic resonance imaging, and will be described with particular reference thereto. It finds application more generally in conjunction with magnetic resonance imaging, magnetic resonance spectroscopy, and other magnetic resonance applications.
In bore-type magnetic resonance scanners, the transmit coil is typically a birdcage-type coil including a plurality of rungs arranged as a cylinder and terminated by end-rings, end-caps, or so forth. Under radio frequency excitation at a magnetic resonance frequency, these coils generate a rotating B1, magnetic field that is substantially homogeneous over an imaging volume inside the birdcage-type coil. In whole-body coils, the cylindrical birdcage coil is typically arranged coaxially with the bore of the magnetic resonance scanner, and excites a large volume. For certain applications, a smaller birdcage coil is designed and arranged to image an anatomical region or other region of interest. For example, a head coil may be sized to fit over a medical patient's head to facilitate brain imaging or other head imaging. A smaller local birdcage coil can provide better electromagnetic coupling with the region of interest, and employs less radio frequency power as compared with a whole-body coil.
The excited magnetic resonance can be collected by the same coil used for the transmit phase (that is, a transmit/receive coil), or can be collected using a dedicated receive coil, such as a surface coil disposed close to the imaging region. In parallel imaging techniques such as SENSE, an array of receive coils are used in parallel, with suitable data processing performed to generate a composite image from the data acquired by the plurality of coils.
Space constraints can make providing a receiving coil array that is separate from the transmit coil problematic. For example, a birdcage head coil leaves little room for an additional array of surface receive coils. In one approach for addressing this problem, the birdcage transmit coil can be selectively configurable as a degenerate coil in which the mesh loops are decoupled. The birdcage coil is typically used as a volume resonator during the transmit phase, and then is re-configured using PIN diode switches or the like as a decoupled array of conductor loops that serve as coils of a SENSE coil array or other parallel imaging receive array. This approach does not provide flexibility in positioning the conductor loops relative to the imaging subject.
Another difficulty with existing radio frequency coils is loading-induced B1 field inhomogeneity. For static B0 magnetic fields greater than about 1 Tesla, inclusion of a region of a patient or other imaging subject inside the coil can substantially distort the B1 field, leading to an inhomogeneous B1 field. This inhomogeneity can be reduced by designing the radio frequency coil using design modeling that accounts for the coil loading. However, the birdcage coil has a limited number of design parameters, such as the number of rungs, coupling reactances between the rungs and end-rings, and so forth, which limits the extent to which birdcage coil design can counteract asymmetric coil loading.
The following contemplates improvements that overcome the aforementioned limitations and others.